HARNESSING EXCESS PHOTON ENERGY IN PHOTOINDUCED SURFACE ELECTRON-TRANSFER BETWEEN SALICYLATE AND ILLUMINATED TITANIUM-DIOXIDE NANOPARTICLES

Photons absorbed by nanocrystalline TiO2 particles at 254 nm are found to be 7.7 times more efficient than those at 366 nm for driving the p hotocatalytic oxidation of salicylate S in aerated aqueous sols. The o ccurrence of this phenomenon is ascribed to the conjunction of (1) sho rt diffusion times of photogenerated carriers to the surface of nanopa rticles, a fact that allows chemical reaction to compete with energy r elaxation, and (2) favorable donor E-0(S-/S-.) redox potential and int erfacial reorganization energy lambda(R) values, which make electron-t ransfer rates peak at energies inside the valence band of TiO2. Master equation kinetic modeling shows that electron transfer from S into hy perthermal valence band holes takes place at rates consistent with k(s c) similar to 10(4) cm s(-1) at optimal exoergicity, if the excess ene rgy is dissipated into the crystal lattice within a few picoseconds. H ydroxyl ions as donors would require much slower thermalization rates.